Significance- Dysfunction of reward and aversion systems figures prominently in affective disorders, pathological pain and addiction. Preference behavior is the most accepted approximation of these processes in non-humans amenable to objective measurement. Knowledge of molecular, genetic markers of neurons dedicated to the above functions, and whether such defined populations exist, offers the exciting prospect of the development of highly-targeted therapies with unprecedented potency and specificity. Thus the long-range goal of the proposal is to identify distinguishing molecular markers of such populations, and the goal of the proposal is to determine whether such populations exist in a vertebrate. Zebrafish, a high-throughput, simple vertebrate model amenable to genetics with genes ~80% similar to mouse, will be used chiefly because they are translucent, allowing use of light to inhibit specific populations of neurons with temporal precision using the newly-developed genetically-encoded chloride pump, halorhodopsin. The wavelength used to trigger halorhodopsin is distinct from that producing an innate preference. The overall hypothesis to be evaluated is that light/dark preference, a non-reflexive choice behavior, is controlled by discrete populations of neurons. While dopaminergic neurons have been implicated in preference behaviors, in mammals the role of these neurons, which release co-transmitters, remains unclear, demanding an unbiased, brain-wide approach as well. In both aims, preference will be assessed in a light/dark choice tank by measuring time spent in light and dark compartments. Aim 1, to determine the developmental time course of the reversal of the motivational valence of light, and then identify neurons wherein the cessation or onset of activity correlates with the reversal of the motivational valence of light, tracks the appearance or disappearance of the activation of neurons as the preference for light reverses sign over development to determine if distinct candidate neuronal populations related to preference exist and identify them. Co-expression of activity-dependent immediate early genes and neurochemical markers will be assessed to track activity and identify cell types. Aim 2, to determine whether activation of dopamine neurons is necessary for light preference, will use cell-type specific expression of halorhodopsin using transgenic fish to suppress neuronal activity in these populations to obtain cause-effect data. PUBLIC HEALTH RELEVANCE: The proposed work seeks to identify brain cells dedicated to controlling whether an animal prefers or avoids something, which should encompass cells related to motivation, enjoyment or aversion. Dysfunction of the systems underlying these processes figures prominently in mood and anxiety disorders as well as pathological pain and addiction.